Composite materials are known, wherein a fibre reinforcing material is impregnated with a resin which is cured to give a strong, lightweight material. Such materials are particularly suitable for structures where the low weight to strength ratio may be exploited, for example in aircraft structures.
Conventionally, some lightweight structures are made of lightweight metal alloys and in some of these structures one or more sheets of the structure are perforate to provide sound attenuation. Such perforate sheets are found, for example, on the face of blocker doors in jet engines. These perforate metal alloy sheets may be made by known techniques of casting and drilling or punching, and the like. Hitherto, the use of resin-impregnated fibre-reinforced composite materials would not have been thought applicable to the manufacture of such perforate sheets because, drilling or punching fibre-reinforced composite materials would have been expected to break the continuity of the fibres with loss of strength. Furthermore, such additional processing is time consuming.
It has been found that by forming the perforations in the reinforcing fibre material before it is impregnated and cured, by means of needles in a mould, the fibres of the reinforcing fibre material may be parted by the needles rather than being broken, thereby maintaining the continuity of the fibres. It is believed that this results in a composite material with superior qualities.
A problem often associated with the manufacture of composite materials with intricate shapes, such as for example, numerous perforations, is that the material may be difficult to remove from the mould. United Kingdom Patent Application No. GB2090183A, for example, relates to a method of manufacturing a perforate composite material wherein a former is melted to release the composite from the mould. Such a process may be time consuming.